Drosophila melanogaster, a genetically well-characterized organism, is especially suitable for the elucidation of the biochemical basis of DNA repair mechanisms in eucaryotes. Since the various modes of DNA repair are intimately related to DNA replication, mutagenesis, and recombination, the enzymes involved in the correction of DNA damages may be instrumental in the carcinogenic process. The proposed research is designed to screen for specific repair endonuclease activities in wild-type Drosophila and in various known repair-deficient mutants. In order to assay for such damage-specific endonucleases, DNA substrates will be prepared by treatment with ultraviolet light, x-rays and alkylating agents. Endonuclease activities will also be investigated at different defined developmental stages of Drosophila, so as to estimate the organism's potential for repair during its life cycle.